The present invention relates to a device for controlling the combustion process in a combustion engine. The invention especially relates to such a device for reducing soot emissions in combustion engines in which the fuel/cylinder gas mixture is ignited by compression heat generated in the cylinder.
Soot particles (or particulates) are a product which, during combustion, can both be formed and subsequently oxidized into carbon dioxide (CO2). The quantity of soot particles measured in the exhaust gases is the net difference between formed soot and oxidized soot. The process is very complicated. Combustion with fuel-heavy, i.e. rich, fuel/air mixture with poor mixing at high temperature produces high soot formation. If the formed soot particles can be brought together with oxidizing substances such as oxygen atoms (0), oxygen molecules (O2), hydroxide (OH) at sufficiently high temperature for a good oxidation rate, then a greater part of the soot particles can be oxidized. In a diesel engine, the oxidation process is considered to be in the same order of magnitude as the formation, which means that net soot production is the difference between formed quantity of soot and oxidized quantity of soot. The net emission of soot can therefore be influenced firstly by reducing the formation of soot and secondly by increasing the oxidation of soot. Carbon monoxide emissions (CO) and hydrocarbon emissions (HC) are normally very low from a diesel engine. Yet the percentages can rise if unburnt fuel ends up in relatively cool regions. Such regions are, in particular, zones with intense cooling located close to the cylinder wall. Another example is cavities between piston and cylinder lining.
Nitrogen oxides (NOx) are formed from the nitrogen content in the air in a thermal process which has a strong temperature dependency and depends on the size of the heated-up volume and the duration of the process.
A combustion process in which the fuel is injected directly into the cylinder and is ignited by increased temperature and pressure in the cylinder is generally referred to as the diesel process. When the fuel is ignited in the cylinder, combustion gases present in the cylinder undergo turbulent mixing with the burning fuel, so that a mixture-controlled diffusion flame is formed. The combustion of the fuel/gas mixture in the cylinder gives rise to heat generation, which causes the gas in the cylinder to expand and which hence causes the piston to move in the cylinder. Depending on a number of parameters, such as the injection pressure of the fuel, the quantity of exhaust gases recirculated to the cylinder, the time of injection of the fuel and the turbulence prevailing in the cylinder, different efficiency and engine emission values are obtained.
Below follows two examples of state of the art arrangements attempting to lower both soot and NOx-emissions by controlling the flame, and trying to brake the well known “trade off” between soot emissions and nitrogen oxide emissions, which is typical of the diesel engine, and which “trade-off” is difficult to influence. The majority of measures which reduce soot emissions increase the nitrogen oxide emissions.
EP1216347 shows an arrangement for controlling the combustion process in a combustion engine by controlling the combustion flame, with the purpose to decrease soot and NOx emissions. The fuel is injected into the combustion chamber with a sufficiently high kinetic energy (high injection pressure) so as to supply kinetic energy to the spray in such way that a spray-internal mixing process and a large-scale global mixing process between fuel and cylinder gas is achieved, thus keeping the soot emissions below a selected level. A proportion of recirculated exhaust gas is selected such that the nitrogen oxide emissions are kept below a selected level.
U.S. Pat. No. 6,732,703 shows an arrangement for minimizing NOx emissions and soot particulates. Here, the fuel spray hits inner bowl floor section during injection in order to cool down the combustion thereby decreasing the creation of NOx. The fuel is injected with high pressure and the piston is shaped to maintain the momentum in the spray plume and fuel/air mixture so that good mixing of available oxygen and soot occurs late in the combustion process. A lot of the momentum is lost when the spray plume hits the inner bowl floor section.
Due to coming future emission legislation for combustion engines there is a need to further lower the soot emission levels in order to meet coming demands.
It is desirable to overcome the deficiencies of the prior art and to provide an internal combustion engine containing a combustion chamber arrangement designed to reduce undesirable soot emissions sufficiently to meet new regulated limits. Thus, it is desirable is to minimize the amount of soot by—promoting efficient combustion of the fuel within combustion chamber by ensuring the complete burning/oxidation of the soot formed during the combustion process.
It is also desirable to provide an engine wherein the shape, position and dimensions of various features of the combustion chamber arrangement, including the piston bowl and the injection spray angle, cause the spray/flame to impinge upon and contact the piston bowl surface in the outer bowl section and that the curvilinear shape of said outer bowl section is such that a reflection angle of said impingement of said central axis is positive during at least start of injection in order to increase balance between tangential and vertical upwardly directed movements of the spray/flame.
It is also desirable to provide a diesel engine capable of operate with significant soot emission improvements compared to e.g. an US02-engine, while also satisfying mechanical design constraints for a commercially acceptable engine.
It is also desirable to provide an engine including a combustion chamber arrangement having dimensions and dimensional relationships to ensure oxidation of sufficient amount of soot during combustion to minimize soot available for discharge to the exhaust system. This can be done without increasing the creation of NOx.
According to an aspect of the invention, an engine with a combustion chamber arrangement having certain predetermined combinations of combustion chamber design parameters, including specific combustion chamber dimensions and dimensional relationships is provided. For example, in the preferred embodiment an engine with a combustion chamber, comprises: an engine body including an engine cylinder, a cylinder head forming an inner face of the combustion chamber and at least one intake port formed in the cylinder head for directing intake air into the combustion chamber with no or low swirling effect during operation; a piston positioned for reciprocal movement in said engine cylinder between a bottom dead center position and a top dead center position, said piston including a piston crown comprising a top face facing the combustion chamber, said piston crown comprising a piston bowl formed by an outwardly opening cavity, said piston bowl comprising a projecting portion having a distal end and an inner bowl floor section extending inwardly at an positive inner bowl floor angle (a) from a plane perpendicular to an axis of reciprocation of the piston, said piston bowl further comprising an outwardly flared outer bowl section having a concave curvilinear shape in cross section; an injector mounted on the engine body adjacent said projecting portion of said piston bowl to inject fuel into the combustion chamber with high injection pressure, said injector comprising a plurality of orifices arranged to form fuel spray plumes, which eventually becomes flames. One preferred embodiment of the invention is characterized in that each of said plurality of orifices has a central axis oriented at a positive spray angle from a plane perpendicular to the axis of reciprocation of the piston sufficient to cause the spray/flame to impinge on said outer bowl section and that the curvilinear shape of said outer bowl section is such that a reflection angle of said impingement of said central axis of said orifices is positive during at least start of injection in order to increase balance between tangential and vertical upwardly directed movements of the spray/flame and to preserve mixing energy late in cycle for increased soot oxidation.
In a further developed embodiment of the invention said injector is arranged to inject fuel with an average injection pressure in the range of 1000 to 3000 bar. In a further preferred embodiment said average injection pressure is in range of 1500 to 2500 bar.
In one further preferred embodiment of the invention said central axis is arranged to impinge said outer bowl section during the whole injection.
In another further preferred embodiment of the invention the curvilinear shape of said outer bowl section has a radius in the dimensionless range of 0.054 to 0.117, and by which figures of the dimensionless range a range in a length measure can be provided for an engine with a specific engine cylinder volume by multiplying said dimensionless figures for said radius with a cylinder volume of said specific engine and elevated by one third. In a more preferable embodiment of the invention said radius is in the range of 0.066 to 0.101. The dimensionless scaling method is further described below.
In another further preferred embodiment of the invention said reflection angle of said impingement is in the range of >0 to 50 degrees when said piston is at a top dead center position. In another preferred embodiment of the invention the reflection angle (γ) is in the range of 10<γ<35 degrees at a top dead center position.
In still another preferred embodiment of the invention said reflection angle of said impingement is in the range of >0 to 50 degrees during start of injection and larger part of said injection and that said reflection angle is slightly negative during end of injection. In another preferred embodiment of the invention the reflection angle (γ) is in the range of 10<γ<35 degrees during start of injection.
In still another preferred embodiment of the invention said reflection angle of said impingement is in the range of >0 to 50 degrees during the whole injection. In another preferred embodiment of the invention the reflection angle (γ) is in the range of 10<γ<35 degrees during the whole injection.
In another preferred embodiment of the invention the number of said orifices are 4 or more, and in a further embodiment 5 to 7.
In another one further preferred embodiment of the invention said swirling effect resulting in a swirl ratio in the range of 0.0 to 0.7.
In another one further preferred embodiment of the invention a geometry of the inner bowl floor section in relation to the spray axis is arranged in such a way so that there is enough volume and distance between the inner bowl floor section and the spray axis (30) so that disturbing contact between the unignited nozzle near portion of the spray and the inner bowl section is avoided.
In another one further preferred embodiment of the invention said injected fuel, when injected, is arranged to form a mixture with said intake air in said combustion chamber, and that said mixture self ignites when compressed by said piston.
In another one further preferred embodiment of the invention said engine is arranged to add a predetermined portion of re-circulated exhaust gas to said intake air, said portion being adapted so that nitrogen oxide emissions emerging from said combustion are kept below a selected low level.
Further advantageous embodiments of the invention emerge from the following detailed description.